1. Field of the Invention
The present invention relates to a dielectric ceramic material that includes lead in the composition thereof, particularly a dielectric ceramic material that can be sintered at a low temperature and a method for producing the same, and an auxiliary oxide that makes it possible to sinter the dielectric ceramic material at a lower temperature.
2. Description of the Related Art
A dielectric ceramic material based on PZT (lead zirconate titanate) is useful as the material to make piezoelectric transformer, piezoelectric actuator or the like, but has drawbacks as the desired composition of a sintered material may not be obtained due to evaporation of lead oxide (PbO) in case the sintering temperature is high, and it is necessary to use an expensive high-melting point metal such as platinum (Pt) as the material to make an internal electrode leading to a high production cost. To circumvent these problems, various methods have been studied for sintering the dielectric ceramic material at a low temperature, which can be roughly divided into the following three groups.                {circle around (1)} Method of adding glass        {circle around (2)} Method of pulverizing calcined powder of dielectric material to decrease the particle size        {circle around (3)} Method of making the melting point of a liquid phase generating oxide of dielectric material lower        
Among these, the method {circle around (1)} that adds glass having a low melting point to the dielectric material so as to lower the sintering temperature has a great effect of lowering the sintering temperature but also decreases the dielectric constant of the dielectric material by adding glass thereto. There is also a problem in that the inclusion of a glass that has lower strength decreases the product strength.
The method {circle around (2)} improves the reactivity of particles by pulverizing the calcined powder of the dielectric material into fine powder by means of a mill or the like, thereby making it possible to sinter the material at a lower temperature, and is disclosed in, for example, Japanese Unexamined Patent Publication No. 7-277822, Japanese Unexamined Patent Publication No. 8-104568 and Japanese Unexamined Patent Publication No. 9-278535. However, sintering temperatures of PZT materials are generally in a range from 1100 to 1200° C., and the decrease in the sintering temperature that can be achieved by method {circle around (2)} is about 100° C. at the most. In order to allow for the use of less expensive silver (metal Ag) as the electrode material, the sintering temperature must be 950° C. or lower. If Pd is added to the Ag, the sintering temperature must be 980° C. or lower. Thus the effect of decreasing the sintering temperature by the method {circle around (2)} is insufficient.
The method {circle around (3)} decreases the melting point of liquid phase generating oxide that forms a liquid phase when sintering the dielectric material. In the case of PZT, for example, the composition of the dielectric material after calcination is a mixture of lead titanate (PbTiO3), lead zirconate (PbZrO3) and lead zirconate titanate (Pb(Ti0.5Zr0.5)O3), and a trace of the PbO that did not contribute to the reaction of generating PbTiO3 and PbZrO3 during calcination forms a liquid phase during the sintering process. This liquid phase is considered to react with PbTiO2 and PbZrO2, through contact therewith, so that the sintering of Pb(Ti0.5 Zr0.5)O3 proceeds. Thus sintering at a low temperature is made possible by adding an oxide that lowers the melting point of PbO in the process of preparing the stock powder, thereby to form the liquid phase at a lower temperature.
However, the method {circle around (3)} cannot control the quantity of the liquid phase generating oxide, namely PbO that did not contribute to the reaction of generate PbTiO3 and PbZrO3. Therefore, there arises such a problem that the sintering temperature varies when there is a slight change in the producing condition. While the liquid phase generating temperature changes in accordance with the ratio of PbO to the low-melting point oxide during sintering, this ratio cannot be controlled and therefore the temperature cannot be controlled to such a level that sintering of the dielectric material is effectively accelerated. In addition, although there will be no problem if the oxide to be added lies in the grain boundary that has no relation to the composition of the dielectric material after sintering, in reality the oxide has a possibility of being included in the composition of the dielectric material and therefore may deteriorate the product performance when allowed to remain in the sintered material in a significant quantity.